The present disclosure relates generally to portable communication devices that communicate with accessory devices using serial protocols and in particular to device-dependent selection by the portable communication device between modes (or roles) of an asymmetric serial protocol.
In recent years, a number of portable communication devices (PCDs) have been developed. Examples of PCDs include, for example, portable media players, mobile phones, personal digital assistants (PDAs), portable e-mail devices, video game players, portable navigation units relying on Global Positioning System (GPS) satellite data, and multi-function devices that can integrate numerous functions such as media storage and playback, mobile phone, Internet access, e-mail, personal information management, game play, GPS/navigation capability, and the like. Examples of multi-function PCDs include various iPhone® and iPod® models manufactured and sold by Apple Inc., assignee of the present application, as well as other portable electronic devices made and sold by other manufactures and distributors under their respective brand names.
PCDs are frequently docked with other electronic devices, referred to herein as “accessories.” For example, from time to time, a user may dock a PCD with a personal computer to synchronize media content and/or metadata, personal data, and the like. A user may at other times dock the same PCD with other electronic devices, such as an in-vehicle media system, a speaker dock, or the like. The user may also dock the PCD with a charger that provides power to the PCD but does not include other data or information sharing capability.
To facilitate communication between PCDs and accessories, it is common to rely on standard point-to-point communication protocols, such as Universal Serial Bus (USB). The USB specification (promulgated by USB Implementers Forum, Inc. and available at their website, www.usb.org) specifies two signal contacts, referred to as D+ and D−, for data transmission and two additional contacts for power, referred to as VBUS and specified as 5.0 volts (V), and ground. In a USB connection, a “USB host” delivers power to the VBUS contact while a “USB device” relies on the USB host to provide the VBUS power.